Add RequestStoreWrap to cache via RequestStore

I don't like the idea of `RequestStore` at all, because it's just a
global state which shouldn't be used at all. But we have a number of
places calling `ProtectedBranch.protected?` and `ProtectedTag.protected?`
in a loop for the same user, project, and ref whenever we're checking
against if the jobs for a given pipeline is accessible for a given user.
This means we're effectively making N queries for the same thing over
and over.

To properly fix this, we need to change how we check the permission,
and that could be a huge work. To solve this quickly, adding a cache
layer for the given request would be quite simple to do.

We're already doing this in Commit#author, and this is extending that
idea and make it generalized.

1 files changed, 113 insertions, 0 deletions

diff --git a/spec/lib/gitlab/cache/request_store_wrap_spec.rb b/spec/lib/gitlab/cache/request_store_wrap_spec.rb
new file mode 100644
index 00000000000..6a95239066b
--- /dev/null
+++ b/spec/lib/gitlab/cache/request_store_wrap_spec.rb
@@ -0,0 +1,113 @@
+require 'spec_helper'
+
+describe Gitlab::Cache::RequestStoreWrap, :request_store do
+  let(:klass) do
+    Class.new do
+      extend Gitlab::Cache::RequestStoreWrap
+
+      attr_accessor :id, :name, :result
+
+      def self.name
+        'ExpensiveAlgorithm'
+      end
+
+      def initialize(id, name, result)
+        self.id = id
+        self.name = name
+        self.result = result
+      end
+
+      request_store_wrap_key do
+        [id, name]
+      end
+
+      request_store_wrap def compute(arg)
+        result << arg
+      end
+
+      request_store_wrap def repute(arg)
+        result << arg
+      end
+    end
+  end
+
+  let(:algorithm) { klass.new('id', 'name', []) }
+
+  context 'when RequestStore is active' do
+    it 'does not compute twice for the same argument' do
+      result = algorithm.compute(true)
+
+      expect(result).to eq([true])
+      expect(algorithm.compute(true)).to eq(result)
+      expect(algorithm.result).to eq(result)
+    end
+
+    it 'computes twice for the different argument' do
+      algorithm.compute(true)
+      result = algorithm.compute(false)
+
+      expect(result).to eq([true, false])
+      expect(algorithm.result).to eq(result)
+    end
+
+    it 'computes twice for the different keys, id' do
+      algorithm.compute(true)
+      algorithm.id = 'ad'
+      result = algorithm.compute(true)
+
+      expect(result).to eq([true, true])
+      expect(algorithm.result).to eq(result)
+    end
+
+    it 'computes twice for the different keys, name' do
+      algorithm.compute(true)
+      algorithm.name = 'same'
+      result = algorithm.compute(true)
+
+      expect(result).to eq([true, true])
+      expect(algorithm.result).to eq(result)
+    end
+
+    it 'computes twice for the different class name' do
+      algorithm.compute(true)
+      allow(klass).to receive(:name).and_return('CheapAlgo')
+      result = algorithm.compute(true)
+
+      expect(result).to eq([true, true])
+      expect(algorithm.result).to eq(result)
+    end
+
+    it 'computes twice for the different method' do
+      algorithm.compute(true)
+      result = algorithm.repute(true)
+
+      expect(result).to eq([true, true])
+      expect(algorithm.result).to eq(result)
+    end
+
+    it 'computes twice if RequestStore starts over' do
+      algorithm.compute(true)
+      RequestStore.end!
+      RequestStore.clear!
+      RequestStore.begin!
+      result = algorithm.compute(true)
+
+      expect(result).to eq([true, true])
+      expect(algorithm.result).to eq(result)
+    end
+  end
+
+  context 'when RequestStore is inactive' do
+    before do
+      RequestStore.end!
+    end
+
+    it 'computes twice even if everything is the same' do
+      algorithm.compute(true)
+      result = algorithm.compute(true)
+
+      expect(result).to eq([true, true])
+      expect(algorithm.result).to eq(result)
+    end
+  end
+end